The invention relates to a hydraulic circuit, and more particularly, to a hydraulic circuit for an active seat suspension system.
Passive suspension systems, such as for a vehicle seat, are known. For example, John Deere production 6000, 7000, 8000 and 9000 Series tractors have passive seat suspension systems which include a hydraulic shock absorber in parallel with an air bag. Active suspension systems are known which include an electro-hydraulically controlled actuator working in parallel with a resilient device, such as a spring. For example, U.S. Pat. No. 4,363,377 (Van Gerpen), issued Dec. 14, 1982, discloses an active seat suspension system with a hydraulic actuator in parallel with a spring. A control system controls fluid communication to the actuator in response to a seat position signal, a stiffness control, a seat height control and a gain control. U.S. Pat. No. 6,000,703 (Schubert et al.), issued Dec. 14, 1999, discloses an active cab or seat suspension control system with a hydraulic actuator in parallel with a pneumatic air spring or air bag. An active seat suspension system which actively controls the seat isolation with hydraulics and an accelerometer is shown in An Active Seat Suspension System For Off-Road Vehicles, by Grimm, et al. With an electro-hydraulic active position control system, hydraulic interactions may occur at vehicle start up and shut down. At start up, hydraulic pressure can be generated before the electronic control is functioning. If the hydraulic circuit is not blocked when electronic control is enabled, sudden movements in the actuator can result from supply pressure being present prior to the proper positioning of the actuator control valve. In the event of loss of electrical power, electronic positioning control could be lost, resulting in undesired action of the suspension system. Typically, a spool position feedback sensor is used to accurately position the actuator control valve spool upon startup. This adds cost and complexity to the system. Accordingly, it is desirable to have a hydraulic circuit which does not require the use of a spool position feedback sensor.
Accordingly, an object of this invention is to provide an active suspension system which prevent undesired hydraulic interactions at vehicle start up and shut down.
A further object of the invention is to provide such an active suspension system wherein action of the hydraulic actuator is prevented until electronic control is enabled.
Another object of the invention is to provide an active suspension system which prevents undesired action of the suspension system if electrical power is lost.
Another object of the invention is to provide an active suspension system with a hydraulic circuit which does not require the use of a spool position feedback sensor.
These and other objects are achieved by the present invention, wherein an active suspension system for supporting a mass, such as a seat on a base of a vehicle, includes a hydraulic actuator coupled between the seat and the base, an pneumatic off-load device between the seat and the base, and a control system which actively controls the hydraulic actuator and which controls the off-load member. A flow control valve controls the extension and retraction of the actuator. A proportional pressure control valve is connected between a primary pressure reducing valve and an inlet of the flow control valve. During start up, the pressure control valve prevents the supply of pressurized fluid to the flow control valve until the flow control valve is driven to its center or null position. During an electrical power failure, the solenoid operated pressure control valve is spring driven to a shut off position preventing communication of pressure to the flow control valve.